Moves the boid simulation forward one tick - if you're running an animation,
you should be calling this on each frame.

flock.boids

All of your boids are stored as an array of arrays, with each
array containing the following variables for a single boid:

[xPosition, yPosition, xSpeed, ySpeed, xAcceleration, yAcceleration]

Because the flock is just an array, it should be entirely safe for you
to add and remove elements without any unintended side effects, provided all
of the arrays are at least 6 elements long and contain numerical values. For
example, you can add a new boid moving at a random speed to the origin like so:

flock.boids.push([0,0,Math.random()*10-5,Math.random()*10-5,0,0])

flock.attractors

You can use attractors to control the flow of the boids - essentially,
providing them with goals and obstacles. Each attractor contains:

[xPosition, yPosition, radius, force]

Note that you can use a negative value for force to repel boids instead of
attracting them. Again, it should be safe to modify, add and remove these
arrays without any surprises.